Internet mail protocol standards dictate precisely what the format of each sender and recipient address can be, and our service will reject any messages sent that do not meet these standards.
Typically, when users receive this error it's due to a syntax error in the recipient's email address (such as omitting the @ symbol). You will need to correct this error before you are able to send messages to that recipient. Please ensure that all recipient addresses match the standards established by RFC5322. You can also read a simplified description of the proper syntax on Wikipedia.
If you have verified the recipient address, believe it is completely valid, and are still receiving an issue, please email email@example.com.